In agriculture, trailed or self propelled units such as sprayers, have booms which are towed behind tractors and are controlled to deliver insecticides and the like to crops through which the tractor drives. Sprayer boom height is typically dynamically positioned in order to maintain a fixed distance between the boom and the crop or soil. Booms are typically rotatable about an axis parallel to the longitudinal axis of the tractor so that they can rotate and remain substantially parallel to the crop even if the tractor drives over sloped ground. Precise control over the height of the boom on which the sprayer nozzles are mounted with respect to the crop is required to avoid damage to the crop by the boom itself as well as to ensure even and proper application of the sprayed chemicals to the crop. The height control of the boom above the crop can be manually or automatically controlled in a number of ways, some of which include the use of ultrasonic distance measuring to determine the height of the boom above the crop. Similarly, in agricultural harvesting operations, cutting blades on a cutter bar must be maintained at a certain height relative to the top of the crop or to the ground level so that crop is harvested accurately even if the crop height or ground level changes with respect to the cutting blades.
In ultrasonic distance measuring, ultrasonic signals are emitted from a transducer and the reflected echoes or return signals from objects in the path of the ultrasonic pulse are detected, after a time interval, by the transducer. The elapsed time between the transmission of the pulse and the receiving of a return pulse reflected off of an object (i.e. an echo) can then be used to calculate the distance to the objects causing each reflected return.
Because it is imperative that the sprayer boom does not come into contact with the top of the crop as the boom is being towed behind the tractor, or that cutting blades do not cut too high or low on a crop, height control systems which use ultrasonic distance measuring have one or more ultrasonic sensors arranged on the sprayer boom or cutter bar which attempt to detect the distance between the boom or cutter bar and some reference datum by listening for the echoes which bounce back to the transducer after the pulse is transmitted.
Using the well known relationship between speed, distance and time, calculating the distance which the ultrasonic sound pulse has travelled before being reflected by objects in the path of the pulse is simple and thus the distance between the boom or cutter bar and a feature causing a reflection can be determined. Once the distance between the boom or cutter bar and a reference datum is known, the sprayer boom or cutter bar can be moved up or down automatically to maintain a predetermined separation distance between the sprayer boom or cutter bar and the reference datum (typically either the top of the crop or the ground level).
Controlling a sprayer boom so that it maintains a fixed distance from the top of the crop can result in a situation in which, in the absence of crops under the sprayer boom, the control system will interpret a reflection of the ultrasonic pulse from the ground under the sensor to be a reflection representative of the top of the non-existent crop because it is the first echo received. If the sprayer boom is being controlled such that a fixed separation between the top of the crop and the sprayer boom is being maintained then this erroneous interpretation of the reflection from the soil as a top of crop signal will cause the sprayer boom to be lowered to maintain the predetermined separation. However, the separation in this case would be between the sprayer boom and the ground level which has been erroneously interpreted as corresponding to the top of the crop.
This temporary absence of the crop in the transmission path of the ultrasonic sensor will result in the sprayer boom being lowered much further than would be the case if a true reflection had been received from the top of the crop. In some cases, the sprayer boom may be lowered to such an extent that part of it contacts the ground. In any case, it is likely the boom will have been lowered below the true level of the top of the crop and thus a collision between the boom and the approaching crop may occur.
If height control with respect to the local ground level is being carried out, such as in the example of a harvester cutter bar, then situations can arise in which a reliable ground level cannot be determined or where, due to either matter resting on the ground or vegetation and the like being present under the sensors, an erroneous ground level is determined. In the first case, a ground level controlled system can become inoperable if it cannot determine a reliable ground level. In the second case in which a ground level is determined which is not the true ground level then the sprayer boom/cutter bar can again be raised or lowered when it should not be.
Clearly, alternative systems and methods of controlling the height of a sprayer boom/cutter bar are required which do not suffer from these problems.